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Geological voids

Voids in the underlying bedrock can be a major problem for developments. Using geophysics can help identify these features and significantly mitigate the associated risk and cost.

Sub-surface voids are a significant risk in some areas, such as where gypsum deposits are present or the bedrock is prone to fracturing and weathering (some limestones and chalk). Geophysics can identify if voids are present beneath a site. Near-surface voids can be detected using ground penetrating radar (GPR) electromagnetics (EM) and in some cases magnetic techniques. If deeper voids (> 3 m to 5 m) are present then these techniques will not work but there are other, less commonly used, techniques that can be effective. Probably the most reliable of these techniques is microgravity. This identifies sub-surface density variations which can be associated with deep voids, areas of ‘loose’ fill material or heavily fractured bedrock.

Phase recently undertook a survey where a desk-based assessment had identified a possible risk of gypsum deposits being present. Gypsum dissolves rapidly if exposed to water and so if gypsum deposits are located beneath a site there is the potential for solution features to be present. These can take the form of voids or, if there has been an historic collapse, fractured bedrock or partially infilled voids. Boreholes cannot be relied upon to detect all solution features that may be present on a site, even with a closely spaced grid of boreholes, as it is possible that even relatively large features could be present in between the boreholes. Another major problem with boreholes is that they can open up water pathways into any gypsum that is present thus potentially making worse or speeding up the gypsum dissolution. Microgravity does not have these risks as it is non-intrusive and the entire site can be surveyed allowing a plot of the sub-surface density changes across the entire site to be created.

At this site two large, clear solution features (the presence of which was not previously known), as well as several smaller possible features, were identified as low gravity anomalies. The presence of these features would have a significant impact on any future development and so the microgravity survey allowed the client to make an informed decision on the viability of their project before they had put a spade in the ground.